1. Field of the Invention
The present invention relates to a heating apparatus for heating a toner image and an image forming apparatus including the same.
2. Description of the Related Art
Film-heating type fixing apparatuses are known as fixing apparatuses that fix a toner image formed on a recording material. Heating is performed by applying a current to a heat generation layer provided in a fixing film, thereby prompting the fixing of the toner. Examples of methods for supplying current to a heat generation layer include a contact power supply method in which a power supply unit and a heat generation layer are in contact with each other, and a non-contact power supply method in which the power supply unit and the heat generation layer are not in contact with each other.
The contact power supply method is a method of supplying power by bringing a brush electrode into contact with a ring-shaped electrode attached to the inner circumferential face of a cylinder-shaped fixing film. With the contact power supply method, the power supply sometimes destabilizes due to wearing in the electrodes caused by rubbing. Also, since several hundred Watts to 1 kiloWatt of power is needed for fixing, electrical discharge sometimes occurs between the electrodes. An oxide layer formed by this electrical discharge sometimes destabilizes the power supply as well.
On the other hand, with the non-contact power supply method (Japanese Patent Laid-Open No. 2002-123113), power is supplied from a primary coil to a secondary coil by magnetically coupling the primary coil and the secondary coil. Accordingly, with the non-contact power supply method, wearing of the electrodes and the oxide layer are not generated in principle, and therefore it is possible to supply power more stably than in the contact power supply method.
Incidentally, the fixing apparatus fixes a toner image not only to a recording material that is wide in the width direction that is orthogonal to the conveyance direction, but also to a recording material that is narrow in the width direction. If recording materials with narrow widths are successively passed through the fixing apparatus, portions of the fixing film that do not come into contact with the recording material (non-paper-feeding regions) are likely to increase in temperature. If wide recording materials are fed through in this state, a high temperature offset is generated and wrinkling occurs in the ends of the recording materials. High temperature offsets cause image unevenness. Furthermore, there is also a risk that only the ends of the fixing film will exceed the heat resistant temperature.
Japanese Patent Laid-Open No. 2012-78453 proposes a fixing apparatus that heats only portions that correspond to the sheet size. Three pairs of electrodes are arranged on one heat generation layer at three different positions in the lengthwise direction. The three electrode pairs correspond to three types of sheet sizes. The electrode pair that is energized is switched by a switch in accordance with the size of the sheet being fed (FIG. 3, paragraph 0049 and the like). The distance from one of the electrodes constituting an electrode pair to the other electrode is designed in accordance with the size of the sheet.
Japanese Patent Laid-Open No. 2012-78453 does not disclose any power supply method. If a non-contact power supply method is employed, a switch for selecting the desired electrode pair out of the multiple electrode pairs will be needed on the secondary coil side. The current that flows in the heat generation body instantly reaches 10 [A]. If the switch on the secondary side is a mechanical switch, the switch needs to have a large size. This means increasing the rotation load for the rotating secondary side.
In the case of using an electronic switch on the secondary side, various circuits are needed. Specifically, a switch element such as a triac is needed between the secondary coil and the heat generation body, and a light receiving element that receives a wireless signal is also needed. Furthermore, a control circuit and a drive circuit for controlling the conduction of the switching element based on the received signal, a rectifying circuit for rectifying the AC output of the coil, a power source circuit unit for generating an operation power source for the control circuit and the like by smoothing out the rectified output, and the like will also be needed. Accordingly, the rotation load on the secondary side increases also in the case of using an electronic switch on the secondary side.